The present invention relates to RF (radio frequency) communication systems, and more specifically, to apparatus and methods for maximizing efficiency of power amplifiers under power backoff conditions.
Wireless communication, such as cell phones for voice and data, has become extremely popular. Currently, several wireless schemes are in use, including GSM (Group Special Mobile), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access). Of these, CDMA appears to be emerging as the standard in the U.S., European and Asian markets. CDMA often requires RF transmissions using both phase and amplitude modulation. The efficiency and power consumption of the power linear amplifiers used to generate an RF signal in either a CDMA cell phone or base station are therefore extremely important.
Use of low efficiency linear amplifiers is detrimental for several reasons. Such amplifiers tend to burn a significant amount of energy which is problematic, particularly in a battery operated cell phone. Power consumption is also problematic in base stations. The heat caused by many low efficiency amplifiers in a base station can cause components to fail, thus reducing reliability. The linearity of the power amplifier is also important. In a base stations where the transmission of multiple signals occurs simultaneously, amplifiers characterized by poor linearity may cause the inadvertent mixing of these signals.
A number of types of amplifier classes can be used in RF communication systems, including Class A, Class AB, Class C, Class E, Class F, and Class D (sometimes referred to as digital amplifiers). Each of these types of amplifiers, however, have significant problems when operating in the RF range. For example, Class A and Class AB amplifiers have very poor efficiency but reasonable linearity. Class C amplifiers are reasonably efficient but are only practical for phase modulation. Similarly Class E, F, and D amplifiers are typically only useful for phase modulation applications. Class E amplifiers have improved power efficiency when compared to C type amplifiers, but large voltage swings at their output limit their usefulness. Class F amplifiers exhibit relatively efficient switching characteristics with a repeating input signal. But with a non-repeating input signal, such as those normally encountered in a cellular phone or base station, the problems caused by harmonics become overwhelming.
Conventional class D amplifiers have linear operating characteristics and are generally highly efficient at lower frequencies but have heretofore been subject to several drawbacks at higher frequencies. Most notably, at higher frequencies such as RF they exhibit switching problems at their output transistors. As these transistors switch on and off rapidly, switching transients including high levels of current and voltage are developed at the output, causing overshoot and undershoot.
Another problem with conventional class D amplifiers when used in communication systems where RF signals are both transmitted and received is the xe2x80x9cleakagexe2x80x9d of energy from the transmit band into the receive band. This may occur if the duplexor or T/R switch at the antenna does not completely isolate the signals received at the communication device from the transmit circuitry within the device.
Most cellular systems today use FDD (Frequency Division Duplexing) to achieve simultaneous transmit and receive capability. This is accomplished by using separate frequency bands for transmitting and receiving. For example, IS-95 CDMA systems in the United States uses 824-849 MHz for transmitting from a mobile station (i.e., upstream transmission) and 869-894 MHz for receiving at the mobile station (i.e., downstream transmission). FDD systems require limits on transmit emissions in the receive band to avoid corresponding degradation of the sensitivity of their own and neighboring mobile receivers. Systems which employ time division duplexing (TDD) also require limits on transmit emissions in the receive band, but typically to a lesser extent.
Generally, conventional switching-mode power amplifiers operate by toggling between two states, namely VCC (+V) and GND (0V) levels. A switching device turns on and off at the switching frequency fsw. A switching device, such as an FET (Field Effect Transistor), includes parasitic capacitance C, and charges and discharges this parasitic capacitance C when it turns on and off at the switching frequency fsw. As a result, a switching device causes switching loss Ploss due to the parasitic capacitance C, which is expressed as:
Ploss=fswxc2x7Cxc2x7V2
Therefore, the switching frequency fsw is important since it directly affects the switching loss Ploss due to the parasitic capacitance C.
Conventional switching amplifiers which keep the switching frequency fsw constant, and change phase only. In such a case, the switching frequency fsw is approximately equal to a radio carrier frequency fc. The switching frequency fsw is independent of the modulating signal amplitude or power delivered to the load. To maintain maximum efficiency under power backoff conditions, the power supply needs to be reduced as a function of output power required. However, in conventional switching amplifiers, the switching frequency fsw is an average frequency, which is dependent on loop parameters and choice of quantizer sampling frequency fs in addition to the radio carrier frequency fc. Thus, the value of fsw is relatively independent of the power output since the amplitude information can be carried by the switching waveform itself. This results in reduction of PAE (Power-Added Efficiency) of the amplifier when the output power is backed off from maximum.
In view of the foregoing, amplifiers and methods capable of maximizing PAE even when the output power is backed off are needed.
According to a specific embodiment of the present invention, an amplifying device for generating an output signal is disclosed. In one embodiment, the amplifying device includes a filter which filters the input signal, thereby generating a filtered signal; a quantizer which quantizes the filtered signal into one of two values, thereby generating a quantized signal; a driver which amplifies the quantized signal, thereby generating the output signal; and a feedback loop which feeds the output signal to the filter. In this embodiment, the quantizer has characteristics in which the quantized signal does not fluctuate between the two values when the input signal is substantially stable.
In a further specific embodiment of the amplifying device, the quantizer has an input offset. In another further specific embodiment of the device, the quantizer has a hysteresis characteristic.
According to another specific embodiment of the present invention, a method for amplifying an input signal to generate output signal includes filtering the input signal, thereby generating a filtered signal; quantizing the filtered signal into one of two values, thereby generating a quantized signal; amplifying the quantized signal, thereby generating the output signal; and feeding the output signal to the filter. In the method for amplifying, the quantized signal does not fluctuate between the two values when the input signal is substantially stable.
In a further specific embodiment of the amplifying method, the quantizer has an input offset. In another further specific embodiment of the amplifying method, the quantizer has a hysteresis characteristic.
The above embodiments of the present invention are advantageous when it is desirable to reduce the switching frequency fsw as a direct function of the reduced output power instead of varying power supply to backoff power.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.